margin

The margin property defines the outermost portion of the box model, creating space around an element, outside of any defined borders.

Margins are set using lengths, percentages, or the keyword auto and can have negative values. Here’s an example:

.box {
    margin: 0 3em 0 3em;
}

margin is a shorthand property and accepts up to four values, shown here:

.box {
    margin:  ||  ||  || 
}

If fewer than four values are set, the missing values are assumed based on the ones that are defined. For example, the following two rule sets would get identical results:

.box {
    margin: 0 1.5em;
}

.box {
    margin: 0 1.5em 0 1.5em;
}

Thus, if only one value is defined, this sets all four margins to the same value. If three values are declared, it is margin: [top] [left-and-right] [bottom];.

Any of the individual margins can be declared using longhand, in which case you would define only one value per property:

.box {
    margin-top: 20px;
    margin-right: 10px;
    margin-bottom: 20px;
    margin-left: 10px;
}

Each of the margin properties can also accept a value of auto. A value of auto basically tells the browser to define the margin for you. In most cases, a value of auto will be equivalent to a value of 0 (which is the initial value for each margin property) or else whatever space is available on that side of the element. However, auto is handy for horizontal centering:

.container {
    width: 980px;
    margin: 0 auto;
}

In this example, two things are done to center this element horizontally within the available space:

• The element is given a specified width
• The left and right margins are set to auto

Without the specified width, the auto values would essentially have no effect, setting the left and right margins to 0 or else to whatever is the available space inside the parent element.

It should also be pointed out that auto is useful only for horizontal centering, and so usingauto for top and bottom margins will not center an element horizontally, which can be confusing for beginners.

Collapsing margins

Vertical margins on different elements that touch each other (thus have no content, padding, or borders separating them) will collapse, forming a single margin that is equal to the greater of the adjoining margins. This does not happen on horizontal margins (left and right), only vertical (top and bottom).

To illustrate, take the following HTML:

<h2>Collapsing Margins</h2>
<p>Example text.</p>

And the following CSS:

h2 {
    margin: 0 0 20px 0;
}

p {
    margin: 10px 0 0 0;
}

In this example, the h2 element is given a bottom margin of 20px. The paragraph element, which immediately follows it in the source, has a top margin set at 10px.

Common sense would seem to suggest that the vertical margin thickness between the h2 and the paragraph would be a total of 30px (20px + 10px). But due to margin collapse, the actual thickness ends up being 20px.

Although collapsing margins may seem unintuitive at first glance, they are actually useful for a few reasons. First, they prevent empty elements from adding extra, usually undesirable, vertical margin space.

Second, they allow for a more consistent approach to declaring universal margins across page elements. For example, headings commonly have vertical margin space, and so do paragraphs. If margins didn’t collapse, headings that follow paragraphs (or vice-versa) would often require resetting the margins on one of the elements in order to achieve a consistent amount of vertical spacing.

Related Properties

• padding

Browser Support

IE6 is prone to the doubled float-margin bug, which can be resolved in most cases by addingdisplay: inline to the floated element.